Cambridge has 13 million across its 3 stations.
It doesn't. The 2024/25 numbers are 10.6m for Cambridge, 1.5m for Cambridge North, and zero for Cambridge South. Furthermore we have to net out the Cambridge-Cambridge North journeys which reduce the total by about 0.6m.
We only have a "first 4 weeks" figure for Cambridge South usage: we have no idea how much of that is abstracted from Cambridge, and not new traffic.
Your 13m figure is purely speculative.
Cambridge’s central station is busier than Newcastle, Sheffield, Bristol Temple Meads etc.
The Cambridge stations have high usage for two main reasons, the two way flows between Cambridge and London, and the short distance commuter flows into Cambridge. Long distance journeys to/from "the north" are a drop in the bucket.
But the huge traffic levels on the A14/A1 show there is a massive market for travel north - it’s just it’s all car based!
The A14/A1 route serves most of East Anglia, not just Cambridge. Furthermore, the A14 and A1 are far more direct than the rail routes to almost everywhere in "the north". Whatever the rail offer, most travel between East Anglia and "the north" is going to be by car.
Leicester and Birmingham aren’t the North
Leicester and Birmingham are nearer than "the north", and still the numbers are small.
While we have 24 cars an hour to Brighton and the South Coast.
Thameslink is primarily for the core stations and Gatwick Airport, not the South Coast.
Cambridge has always had a direct service to Norwich. It was just very infrequent (just one or two trains daily) between the early 1990s and 2002 when the SRA funded the Anglia Railways bid for a regular link using four newly purchased 2-car 170s.
Compared to the cities of the north, Norwich (and Ipswich) have a very different relationship with Cambridge. For them, Cambridge is one of their main destinations, whereas the northern cities are much more interested in trains to and from other northern cities than they are in connections to Cambridge. That's why the north has the Trans Pennine Upgrade.
You seen to have an aversion to increased services from Cambridge to the North and Midlands
What I look at is how the railway can best support economic growth in and around Cambridge, within the constraints of the existing infrastructure. Going north from Cambridge there is extremely limited capacity, notably at Ely, but also between Ely and Peterborough. The priority for using that scarce capacity is for commuting into Cambridge from stations within Cambridgeshire and Peterborough, on the Fen Line, and on the Breckland line. In the words of the current government, it is bringing Cambridge's growth to the postcodes on those routes. What I have an aversion to is wasting that scarce capacity on long distance services that will bring much less benefit to Cambridge, and at the same time resulting in a sub-optimal service for the commuters into Cambridge that are going to generate the economic growth.
Furthermore, the top priority for any increased capacity between Ely and Peterborough is going to be freight traffic for building the Reservoir in the Fens near Chatteris. Economic growth will grind to a halt if we run out of water.
How much of that explains the difference with Oxford's service, given Oxford historically had Paddington to Birkenhead services?
This isn't correct. The Paddington-Birkenhead service ran via High Wycombe.
Assuming that's from the population of Cambridge and South Cambridgeshire combined
The population of Cambridge and South Cambridgeshire combined was 308k in the 2021 census. This is the area that will be covered by the Greater Cambridge Development Corporation. Cambridge North is in South Cambridgeshire, as are Waterbeach, Shelford, Whittlesford, Foxton, Shepreth, Meldreth and Ashwell.